SKEPTOID BLOG:

Jupiterís Galilean Moons: Callisto

Callisto [kuh-lis-toh] is the eighth satellite from Jupiter and has the oldest landscape in the entire Solar System at about 4 billion years old. It is the only body that is bigger than 1,000 kilometers in diameter in the Solar System that has no evidence of geological resurfacing since even the oldest impact structures.

Above Image: Bright scars on a darker surface testify to a long history of impacts on Jupiter's moon Callisto in this image of Callisto from NASA's Galileo spacecraft.

Image Credit: NASA/JPL/DLR

Name & Discovery:

Callisto was discovered in 1610 by Galileo Galilei along with Io, Europa, and Ganymede. He called them "The Medicean Stars" after the wealthy Medici family. Fellow astronomers Simon Marius and Johannes Kepler discussed the idea of naming the moons after the mythical loves of Jupiter, but the idea didn't catch on for a couple hundred years. Most astronomers called them I, II, III and IV in the order of distance from Jupiter. In the 17th century we were discovering so many moons of Jupiter and Saturn that astronomers finally started calling them the familiar mythological names we know now. These were the first moon, other than the Earth's, to be discovered orbiting around a planet. This discovery provided evidence for the development of the understanding that planets orbit the Sun rather than the Earth.

Above Image: This montage shows the best views of Jupiter's four large and diverse "Galilean" satellites as seen by the Long Range Reconnaissance Imager (LORRI) on the New Horizons spacecraft during its flyby of Jupiter in late February 2007. The four moons are, from left to right: Io, Europa, Ganymede and Callisto. The images have been scaled to represent the true relative sizes of the four moons and are arranged in their order from Jupiter.

It takes Callisto about 16.7 Earth-days to complete an orbit around Jupiter.

Because Callisto is tidally locked, the satellite's rotation period is also 16.7 Earth-days.

Callisto is about 4,800 kilometers in diameter, making it the third largest satellite in the Solar System.

Callisto is 99% as big as Mercury but only about 1/3 it's mass.

The mean surface temperature of Callisto is -139.15 C (-218.47 F).

Callisto orbits beyond Jupiter's main radiation belt.

Above Image: These four views of Jupiter's second largest moon, Callisto, highlight how increasing resolutions enable interpretation of the surface. In the global view (top left) the surface is seen to have many small bright spots, while the regional view (top right) reveals the spots to be the larger craters. The local view (bottom right) not only brings out smaller craters and detailed structure of larger craters, but also shows a smooth dark layer of material that appears to cover much of the surface. The close-up frame (bottom left) presents a surprising smoothness in this highest resolution (30 meters per picture element) view of Callisto's surface.Image Credit: NASA/JPL/DLR

Exploration:

Pioneer 10 was the first mission to return pictures of the Jovian moon. Voyager 1 and Voyager 2 were the first to deliver spectacular, high-res pictures of the satellite, but Galileo has gathered the most data in its flybys past Callisto. Right now, Juno is en route to Jupiter and will arrive in 2016. Much like Cassini, the orbiter's primary objective is to learn about the parent planet but the close proximity will surely provide countless opportunities to observe and gather data from the Jovian satellites. The European Space Agency is also planning to launch an orbiter, JUICE, in 2022 that will spend at least three years making detailed observations and gathering data about Jupiter and the four Galilean Moons.

Above Image: This moderately high resolution view of Jupiter's icy moon, Callisto, shows two, probably related, phenomena that were quite surprising to planetary scientists. First, a dark, mobile blanket of material covers Callisto's surface. Movement of this material occurs on slopes, as seen here on some crater walls. Second, while Callisto has a significant number of large craters, it lacks the related number of small craters which are seen in the crater size distributions of other similar bodies in our solar system. Small craters near slopes would become filled in by the downward movement of the dark material, but what erases the other small craters? One alternative is that the population of potential impactors around Jupiter has fewer small objects than previously expected.

Image Credit: NASA/JPL

Physical Characteristics:

Callisto is thought to be about half silicate rock and half water ice, making it the least dense of the Galilean moons. There's also evidence for carbon dioxide, sulfur dioxide and ammonia on the Jovian moon's surface.

Above Image: This artist's concept, a cutaway view of Jupiter's moon Callisto, is based on data from NASA's Galileo spacecraft which indicates a salty ocean may lie beneath Callisto's icy crust.

Image Credit: NASA/JPL

Callisto's internal structure is expected to consist of a metallic core inside of a layer of rocky material called the mantle. Above the rocky mantle is most likely an icy layer that may or may not also contain rocky material. The metallic core is thought to generate enough heat to melt the ice between the mantle and the icy crust, possibly creating a subsurface ocean.

Callisto doesn't have a magnetosphere of its own, but it alters Jupiter's magnetosphere as it passes through. We don't have all the answers, yet, but this phenomenon is among the evidence for a subsurface ocean existing on the satellite.

Above Image: This picture of a multi-ring basin on Callisto was taken the morning of March 6, 1979, from a distance of about 200,000 km. The complicated circular structure seen at left center is similar to the large circular impact basins that dominate the surface of the Earth's moon and also the planet Mercury.

Image Credit: NASA/JPL

The surface of Callisto is dominated by impact structures of various sizes. Being the oldest landscape in our Solar System, Callisto has more craters than any other planet or moon.

"Callisto's atmosphere is so tenuous that the carbon dioxide particles are literally drifting around without bumping into one another," said Dr. Robert Carlson of NASA's Jet Propulsion Laboratory, Pasadena, CA, principal investigator for Galileo's near-infrared mapping spectrometer instrument.

The thin atmosphere gets swept away from the planet by Jupiter's radiation, so something has to be producing the carbon dioxide from the surface.

There are places that have erosion patterns that suggest there might be carbon dioxide vents on the surface of Callisto.

Mythology:

In the myth, Callisto is a favored attendant of the goddess of the hunt, Artemis. Since Artemis is a chaste goddess, Callisto also upholds her virginity until Zeus (The greek version of Jupiter) become enamored by her innocent beauty and tricked her into sleeping with him. She became pregnant and was cast out by Artemis and had her son, Arcas, alone. In a fit of jealousy, Hera transformed Callisto into a bear. As an adult, Arcas comes into contact with the bear version of his mother and almost kills her. Zeus feels sorry for their situation, so he puts the mother and the son in the sky as the constellations Ursa Major and Ursa Minor. Hera did not like this compromise, so she made it so that the two (in the sky) were unable to dip below the horizon to get a drink or take a bath to punish them (explaining why the constellations can be seen year-round).

Astrophotography Photo of the Day (APOD) Images of Callisto:

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